Many polymeric materials are foamed to provide low density articles such as films, cups, food trays, decorative ribbons, and furniture parts. For example, polystyrene beads containing low boiling hydrocarbons such as pentane are formed into lightweight foamed cups for hot drinks such as coffee, tea, hot chocolate, and the like. Polypropylene can be extruded in the presence of blowing agents such as nitrogen or carbon dioxide gas to provide decorative films and ribbons for package wrappings. Also, polypropylene can be injection molded in the presence of these blowing agents to form lightweight furniture parts such as table legs and to form lightweight chairs.
Polyesters such as poly(ethylene terephthalate) have a much higher density (e.g. about 1.3 g/cc) than other polymers. Therefore, it would be desirable to be able to foam polyester materials to decrease the weight of molded parts, films, sheets, food trays, and the like. Such foamed articles also have better insulating properties than unfoamed parts. However, it is difficult to foam such polyester materials because of the low melt viscosity and low melt strength of typical poly(ethylene terephthalate) and related polyester polymers. The low melt viscosity and low melt strength of the polyesters is a problem because the polymer melt will not adequately retain the bubbles of an expanding gas. It would be desirable therefore to be able to provide polyester polymers which could be foamed with conventional foaming systems.
One approach to providing polyesters having high melt viscosities involves treating preformed polyesters with multifunctional carboxylic acids or polyols to provide branched polyesters. Such compositions are disclosed in U.S. Pat. Nos. 4,132,707; 4,145,466; 4,999,388; 5,000,991; 5,110,844; 5,128,383; and 5,134,028. The branching agents used include tri- and tetracarboxylic acids and anhydrides such as trimesic acid, pyromellitic acid, and pyromellitic dianhydride or polyols such as trimethylolpropane and pentaerythritol. These branching agents will provide polyesters with increased melt viscosities but their use is often disadvantageous. For one thing, the branching agent cannot be put into the initial reaction mixture of polyester reagents because this will lead to crosslinked structures. If added after the polyester is formed, an additional processing step is required and the branching action is hard to control. For example, the branching reaction may proceed too far, providing crosslinked materials which are intractable.
It has now been found that a wide range of polyester compositions can be modified with small amounts of a dicarboxylic acid sulfomonomer to provide copolyester compositions with increased melt viscosities which have good foaming characteristics. Polymers suitable for foaming must have a melt viscosity which is sufficient to retain the bubbles of an expanding gas during molding or extrusion operations. Good melt viscosity is also essential for the manufacture of rigid foams and foamed containers having uniform wall thickness.
Polyesters containing dicarboxylic acid sulfomonomers are disclosed in U.S. Pat. Nos. 3,734,874; 3,779,993; 4,335,220; 4,233,196; 3,853,820; and 5,053,482. U.S. Pat. Nos. 3,734,874 and 4,233,196 are concerned with amorphous water dispersible polyesters which contain at least 8 mole percent of a sulfomonomer and substantial amounts of a difunctional glycol component. U.S. Pat. No. 3,853,820 describes an amorphous water dissipatable polyester with at least 20 mole percent of a poly(ethylene glycol) which is a condensation polymer of ethylene glycol. In addition, U.S. Pat. No. 5,053,482 describes fiber and film forming polyesters based on polyethylene terephthalate polyesters containing 20 to 40 mole percent of diethylene glycol for use in disposable products such as disposable diapers. In contrast, the polyesters of the present invention are crystalline hydrophobic polyesters containing a diol component which consists essentially of ethylene glycol, 1,4-butanediol, 1,6-hexanediol, or 1,4-cyclohexane-dimethanol.
Other patents of interest include U.S. Pat. Nos. 4,499,262; 4,579,936 and U.S. patent application Ser. No. 848,469 filed Mar. 9, 1992. These references are concerned with bottle polymer compositions and do not anticipate their use in foamable compositions.